Composite video signals of the type used in television systems include video and synchronizing information. The video information is divided into sequential trace and retrace signals with the synchronizing information transmitted as pulses during the retrace portion of the video information. During the trace interval, the video signal varies between black and white levels. The retrace interval video signal includes a blanking portion which is at or slightly greater than black level so that the electron beam in the cathode ray tube (CRT) is turned off during retrace. If all of the levels of the various components of the video signal are properly adjusted at the studio and properly transmitted and received, the video signals can be properly displayed on a CRT screen. Unfortunately, adjustment errors and transmitting and receiving errors require processing and correction of the video signal to achieve a satisfactorily displayed image.
Some of the more common errors in the received signal include improper black and white levels and improper synchronizing pulse amplitude. Since the automatic gain control in typical television receivers sets-up on the synchronizing pulse amplitude, improper synchronizing pulse amplitude can cause the received signal to be amplified more or less than desired. If the black level of the trace interval signal is set improperly with respect to the blanking level, the displayed image will be too dark or too light depending upon the direction of the error. Similarly, an improperly set white level will result in improper contrast assumming black level is set correctly. In typical television receivers, the contrast and brightness controls can be used to correct, at least in part, for such errors. Adjustments of the contrast and brightness control to correct erroneous composite video signals, however, cannot correct all errors and necessitates frequent readjustment to provide a properly displayed image.
The signals received by a television receiver are typically provided by a variety of sources each of which can have different errors in signal levels necessitating readjustment of the contrast and brightness controls to produce a properly displayed image. For example, when channels are changed, any signal level errors will necessarily be different. Also, on the same channel the signal may be provided by a variety of sources in the studio such as different cameras, video tape, movie film, and the like. Since each source can have its own unique characteristics, switching from one source to another can alter the signal levels. Compensation for the different signal levels of the various studio signal sources is generally incomplete. Accordingly, the composite video signal received by a television receiver may also include undesired variations in signal levels.
Prior art television receivers have included various forms of automatic control circuitry in an attempt to compensate for such undesired contrast and brightness variations. One prior art form of black level control includes the so-called "back porch clamp" which clamps the black level of the video signal to the blanking level. If the blank level of the video signal is improperly set with respect to the blanking level, however, this form of automatic black level control does not correct the signal properly.
Another form of automatic contrast and brightness control involves detecting and clamping to the blackest and whitest portions of the received signal. While such circuitry circumvents the primary disadvantages of back porch clamping, under some signal conditions the displayed image is improper. For example, on scenes where there is no black in the image, the control circuitry can improperly clamp to a portion of the image not intended to be black and cause those portions to be black. Similarly, in a dark scene with no white level signals intended, the lightest part of the image can be clamped to white level thereby distorting the intended and desired image. Control circuitry of ths type generally also requires peak detecting and sample and hold circuits. Since such circuits will tend to set-up on blanking interval signals, a blanking generator which anticipates the blanking intervals is necessary. Providing such circuitry requires substantial expense and circuit complexity for proper operation.
Other forms of automatic contrast and brightness control circuitry have been used; however, known prior art circuitry of this type, including the types mentioned above, has numerous deleterious effects. For example, the compensation for video signal errors can be incomplete or incorrect under varying signal conditions. Additionally, some forms of prior art circuitry are unduly complex and expensive or require compromises which deleteriously effect the displayed image.
Another form of video signal error which commonly occurs is degradation of the sharpness of the image. Numerous variations in picture sharpness and numerous causes for such variations are encountered in typical TV transmissions. Such variations can be caused, for example, by differing video signal transient characteristics of the various video signal sources used in the TV studio. Also, the video distribution systems used in the studio may have different video frequency response and group delay characteristics. Transmitters and modulators at different studios or stations also have different video frequency response and group delay characteristics. Multipath or ghost pick-up conditions at the receiving antenna can greatly vary detected video signal transient response due to signals being cancelled at some frequencies and reinforced at others. Additionally, the TV receiver radio frequency intermediate frequency, and video responses may vary from channel to channel. Factors other than those mentioned above may also cause variations to picture sharpness.
Two somewhat related forms of compensation for variations in picture sharpness are known in the prior art. One form is generally called video peaking which is a form of high frequency emphasis. Typical prior art video peaking circuits boost the high frequency components of the video signal. Such circuits also typically have an undesired phase delay associated therewith.
The term "aperture correction" is primarily used with cameras to refer to compensation for spot size. The term is also used in receivers to refer to a particular form of video peaking which does not have a phase delay associated therewith. Aperture correction involves the addition of pre-shoot and over-shoot components to transitions or transients in the video signal. Thus, the sharpness of the displayed image is enhanced by sharpening the transition from one brightness level to another brightness level.
While numerous forms of aperture correction and peaking circuitry are known in the prior art, such circuitry typically suffers from one or more disadvantages. Discrete aperture correction circuitry is typically simple to avoid expense but has less than optimum performance. Other known forms of aperture correction circuitry are unduly complex. In addition, known prior art aperture correction circuitry tends to emphasize noise thereby deleteriously affecting the weak signal performance of the television receiver. Accordingly, the expense, complexity, and deleterious performance of known prior art aperture correction circuitry has limited the utility of such circuitry.